High power RF amplifiers are employed in many applications. For example, these amplifiers are used in radar, communication, avionics, electronic counter measures (ECM), medical, and other applications. These amplifiers are generally implemented as hybrid devices, and include one or more discrete transistors, one or more discrete capacitors, and one or more discrete inductors that are housed in a package. The one or more capacitors are typically realized using silicon metal oxide semiconductor (MOS), silicon metal insulator metal (MIM), or ceramic MIM components. The one or more inductors are typically realized using wirebonds.
The transistors used in high power RF amplifiers are typically comprised of a plurality of transistor cells coupled in parallel. The cells are configured to respectively receive and amplify portions of an input RF signal, wherein the individual amplified signals are combined at the transistor output to generate an output RF signal. Because there may be many transistor cells coupled in parallel, a typical RF power transistor has a low input impedance (e.g., 0.075 Ohm for each side of a 130 Watt device), as well as a low output impedance.
Typically, the characteristic impedance of transmission lines for providing RF signals to and from high power RF transistor is usually in the range of 50 to 100 Ohms. Accordingly, RF power devices typically incorporate one or more input matching networks coupled to the input of the transistor, and one or more output impedance matching networks coupled to the output of the transistor. This is done to increase the inherently low transistor die impedances and facilitate matching the amplifier to the 50 to 100 Ohm system impedance used by external components.
A typical input (as well as output) impedance matching circuit is comprised of inductive and capacitive elements. Accordingly, its impedance matching performance is a function of frequency. In the past, acceptable broadband performance using such impedance matching networks was typically difficult to achieve. For example, in a prior 130 Watt amplifier, it was difficult to achieve a desirable input return loss across a band of 2.7-3.5 GHz using the standard input matching networks. Thus, one design was configured for acceptable input return loss in the range of 2.7-3.1 GHz, and another design was configured for acceptable input return loss in the range of 3.1-3.5 GHz. However, having two distinct designs generally complicates parts inventory, manufacturing and other processes.
Accordingly, there is a need for an input impedance matching network that provides desirable input return loss across a wider bandwidth.